I think Heinlein's second-best (after JOB) and vastly underrated commentary on religion was Revolt in 2100, in which a charismatic televangelist is elected President in 2012 ( ) and turns the country into a theocracy. The story is about a not particularly bright chum serving in the Prophet's personal guard is led out of religion and into the secular revolutionary conspiracy by an apostate priestess. It's really quite good, and gives a good perspective from the point of view of a typical Christian. Most of Heinlein's most well-known religion quotes come from Revolt in 2100, actually.

Here's some general Heinlein religion quotes I like.

Sin lies only in hurting other people unnecessarily. All other sins are invented nonsense.

Theology is never any help; it is searching in a dark cellar at midnight for a black cat that isn't there.

Whores perform the same function as priests, but far more thoroughly.

When any government, or any church for that matter, undertakes to say to its subjects, 'This you may not read, this you may not see, this you are forbidden to know,' the end result is tyranny and oppression, no matter how holy the motives. Mighty little force is needed to control a man whose mind has been hoodwinked; contrariwise, no amount of force can control a free man, a man whose mind is free. No, not the rack, not fission bombs, not anything--you can't conquer a free man; the most you can do is kill him.

I've never understood how God could expect His creatures to pick the one true religion by faith - it strikes me as a sloppy way to run a universe.

The Bible is such a gargantuan collection of conflicting values that anyone can prove anything from it.

The Ten Commandments are for lame brains. The first five are solely for the benefit of the priests and the powers that be; the second five are half truths, neither complete nor adequate.

God is omnipotent, omniscient, and omnibenevolent - it says so right here on the label. If you have a mind capable of believing all three of these attributes simultaneously, I have a wonderful bargain for you. No checks, please. Cash and in small bills.